Alkali metal generating agent, alkali metal generator, photoelectric surface, secondary electron emission surface, electron tube, method for manufacturing photoelectric surface, method for manufacturing secondary electron emission surface, and method for manufacturing electron tube

ABSTRACT

The present invention relates to an alkali metal generating agent and others for formation of a photo-cathode or a secondary-electron emitting surface capable of stably generating an alkali metal. The alkali metal generating agent is used in formation of a photo-cathode for emitting a photoelectron corresponding to incident light, or in formation of a secondary-electron emitting surface for emitting secondary electrons corresponding to an incident electron. Particularly, the alkali metal generating agent contains at least an oxidizer comprising at least one vanadate with an alkali metal ion as a counter cation, and a reducer for reducing the ion. An alkali metal generating device comprises at least the alkali metal generating agent and a case housing it, and the case is provided with a discharge port for discharging the vapor of the alkali metal.

TECHNICAL FIELD

The present invention relates to an alkali metal generating agent, analkali metal generating device, a photo-cathode, a secondary-electronemitting surface, an electron tube, a method of production of thephoto-cathode, a method of production of the secondary-electron emittingsurface, and a method of production of the electron tube.

BACKGROUND ART

The known photo-cathodes for emitting an electron (photoelectron orprimary electron) corresponding to an incident photon include so-calledtransmission type photo-cathodes formed on a transparent substrate, andso-called reflection type photo-cathodes formed on a metal substratesuch as Ni, and such photo-cathodes are adopted as important components,for example, in electron tubes such as photomultiplier tubes,photo-tubes, image intensifiers, streak tubes, and so on.

Many of the photo-cathodes now in practical use are made from aphotoelectron emitting material containing an alkali metal (primarily,an intermetallic compound or a compound semiconductor), e.g., anintermetallic compound of Sb and Cs.

Conventionally, the photoelectron emitting material containing the abovealkali metal as a constituent element is formed by generating the vaporof the alkali metal in an ambience held at a predetermined vacuum(preferably, 10⁻⁷-10⁻² Pa in terms of partial pressure of residual gas)and temperature and reacting the alkali metal vapor with a constituentmaterial of the photoelectron emitting material that is to react withthe alkali metal. In an example of forming the photoelectron emittingmaterial of the intermetallic compound of Sb and Cs, for example, adeposited film of Sb, which is a constituent material of thephotoelectron emitting material to react with the alkali metal, is firstformed on a substrate, and the vapor of Cs is then generated to react Cswith the deposited film of Sb, thereby forming a layer of theintermetallic compound.

In this case, the alkali metal is extremely instable in the atmosphereand therefore the alkali metal itself cannot be used as a source of thevapor of the alkali metal. It is thus common practice to use a supplysource (so called an alkali source or alkali metal source) containing asa constituent a combination of an oxidizer with a reducer capable ofgenerating the alkali metal by oxidation-reduction (redox) reaction atpredetermined temperature. Examples of this supply source usedheretofore include powder alkali metal sources, and alkali metal sourcespressure-formed (pressed) in a pellet form. In the presentspecification, the alkali metal source (supply source) for the alkalimetal vapor containing the aforementioned oxidizer and reducer will bereferred to as an alkali metal generating agent.

These powder alkali metal generating agents or pelletized alkali metalgenerating agents are normally used in a state in which the generatingagent is housed in a metal case provided with an aperture enough todischarge the alkali metal vapor to the outside. Furthermore, this metalcase is also used as enclosed in a glass ampule in certain cases. Thenthis metal case is heated in formation of the photo-cathode to generatethe alkali metal vapor.

Furthermore, the alkali metal generating agent is also used, forexample, in formation of the secondary-electron emitting surface ofdynodes in photomultiplier tubes.

An example of such alkali metal generating agents used conventionally isa powdered or pelletized alkali metal generating agent containing Si,Ti, Al, or the like as a reducer and containing as an oxidizer achromate with an alkali metal ion as a counter cation (e.g., Cs₂CrO₄ orthe like). The alkali metal generating agent containing this oxidizer isdisclosed, for example, in Japanese Patent Applications Laid-Open No.55-78438 and Laid-Open No. 53-124059.

DISCLOSURE OF THE INVENTION

The Inventors investigated the above conventional technology and foundthe problems as described below. In a case where the photo-cathode to beapplied to the aforementioned electron tube is produced using the alkalimetal generating agent containing as an oxidizer the chromate with analkali metal ion as a counter cation, the redox reaction between theoxidizer of the above chromate and the reducer has very large reactionrates and, as the temperature of the reaction field gradually increases,the reaction suddenly proceeds at a predetermined temperature enough forprogress of reaction. Therefore, there was a problem in production thatit was extremely difficult to control the reaction rates by control ofreaction temperature once the reaction started.

More specifically, since the temperature of the reaction field quicklyincreases with the sudden progress of the redox reaction, there was apossibility of rupture of the alkali metal generating agent itself, orthe metal case or glass ampule housing the alkali metal generatingagent. If this situation occurs during production of the photo-cathodein the electron tube, it will be difficult to control the amount of thealkali metal and desired performance will not be achieved. In this case,the used metal case is left in a housing of the electron tube such as aglass container, from constraints on production efficiency or the like,and the rupture of the metal case can result in a defective product inappearance.

Furthermore, generating rates and yields of the alkali metal largelyvary because of the sudden progress of the redox reaction, which posed aproblem that states of deposition of the alkali metal were nonuniform inan area where the photo-cathode should be formed or in an area where thesecondary-electron emitting surface of the dynode should be formed. Forexample, in a case where the alkali metal generating agent is heated bya high-frequency heating method, the redox reaction suddenly proceedswith use of the conventional chromate, so that the timing of a stop ofheating cannot be always constant. Therefore, there can occur variationin spectral response characteristics (radiant sensitivity and quantumefficiency) among a plurality of photo-cathodes produced under similarconditions, and, as to a plurality of dynodes produced under similarconditions, there can also occur variation in multiplication efficiencyamong them, so as to result in producing defective products, therebyreducing production efficiency.

The present invention has been accomplished in order to solve theproblems as described above, and an object of the present invention isto provide an alkali metal generating agent for formation of aphoto-cathode or secondary-electron emitting surface capable of stablygenerating an alkali metal, an alkali metal generating device comprisingthe alkali metal generating agent and enabling easy control ofgenerating rates of the alkali metal, a photo-cathode with satisfactoryspectral response characteristics, a secondary-electron emitting surfacewith satisfactory multiplication efficiency, and an electron tube withsatisfactory photoelectric conversion characteristics. Another object ofthe present invention is to provide a method of production of thephoto-cathode, a method of production of the secondary-electron emittingsurface, and a method of production of the electron tube easy information and excellent in reproducibility of performance.

The Inventors conducted elaborate research in order to achieve the aboveobjects and found out that one of significant reasons why the reactionrates of the redox reaction between the aforementioned conventionaloxidizer and reducer were high was not the reducer but rather that theoxidizer of the chromate with an alkali metal ion as a counter cationhad a very strong oxidizing power.

Then the Inventors checked oxidizers with oxidizing power weaker thanthat of the aforementioned chromate and found that, by using a vanadateas the oxidizer, it was feasible to produce a photo-cathode and asecondary-electron emitting surface with performance comparable to thoseproduced using the aforementioned conventional chromate, withoutdifficulty and with good reproducibility.

Namely, an aspect of the present invention is an alkali metal generatingagent serving as a supply source of an alkali metal used in formation ofa photo-cathode for emitting a photoelectron corresponding to incidentlight or in formation of secondary-electron emitting surface foremitting secondary electrons corresponding to an incident electron, thealkali metal generating agent comprising at least an oxidizer and areducer. Particularly, in the alkali metal generating agent, theoxidizer comprises at least one vanadate with an alkali metal ion as acounter cation. The reducer initiates a redox reaction with the oxidizerat a predetermined temperature to reduce the alkali metal ion.

Since the vanadate with the alkali metal ion as a counter cation has anoxidizing power weaker than the aforementioned chromate, the redoxreaction with the reducer proceeds moderately as compared with the caseof the oxidizer being the chromate. For this reason, it is easy tocontrol the reaction rates by adjustment of reaction temperature evenafter the reaction starts once. In other words, the alkali metal (thealkali metal vapor) can be generated on a stable basis, withoutrupturing the alkali metal generating agent itself according to thepresent invention or without rupturing a case housing it.

Accordingly, by using the alkali metal generating agent comprising thisvanadate, it is feasible to produce the photo-cathode with satisfactoryspectral response characteristics or the secondary-electron emittingsurface with satisfactory multiplication efficiency, without anydifficulty and with good reproducibility.

An alkali metal generating device according to the present inventiongenerates an alkali metal used in formation of a photo-cathode foremitting a photoelectron corresponding to incident light or in formationof a secondary-electron emitting surface for emitting secondaryelectrons corresponding to an incident electron. The alkali metalgenerating device comprises a case, a supply source, and a dischargeport. Particularly, in the alkali metal generating device, the case ispreferably a metal case for housing the supply source. The supply sourceis an alkali metal generating agent of the aforementioned structure (thealkali metal generating agent according to the present invention), whichcomprises a raw material to generate the alkali metal. The dischargeport is provided in the case so as to discharge a vapor of the alkalimetal generated in the supply source, from an interior space of the casehousing the supply source, toward the outside of the case.

With the alkali metal generating device of the present inventioninternally housing the alkali metal generating agent of theaforementioned structure, it is feasible to stably discharge the alkalimetal (the alkali metal vapor) generated by the redox reaction betweenthe oxidizer and the reducer in the alkali metal generating agent, fromthe discharge port of the case to the outside.

Accordingly, by using the alkali metal generating device according tothe present invention, it is feasible to produce the photo-cathode withsatisfactory spectral response characteristics or the secondary-electronemitting surface with satisfactory multiplication efficiency, withoutany difficulty and with good reproducibility.

A photo-cathode according to the present invention comprises an alkalimetal which emits a photoelectron corresponding to incident light. Thisalkali metal is the alkali metal generated from the alkali metalgenerating agent according to the present invention. This alkali metalmay be the alkali metal generated from the alkali metal generatingdevice according to the present invention. In either case, by using thealkali metal generating agent or the alkali metal generating device, itis feasible to obtain the photo-cathode with satisfactory spectralresponse characteristics.

A secondary-electron emitting surface according to the present inventioncomprises an alkali metal which emits secondary electrons correspondingto an incident electron. This alkali metal may be the alkali metalgenerated from the alkali metal generating agent according to thepresent invention or may be the alkali metal generated from the alkalimetal generating device according to the present invention. In thismanner, by using the alkali metal generating agent or the alkali metalgenerating device, it is feasible to construct the secondary-electronemitting surface with satisfactory multiplication efficiency. Theincident electron to the secondary-electron emitting surface alsoembraces a photoelectron emitted from the photo-cathode.

Furthermore, an electron tube according to the present invention is anelectron tube having a photo-cathode which emits a photoelectroncorresponding to incident light, and the photo-cathode according to thepresent invention can be applied to the photo-cathode of the electrontube.

When the electron tube comprises the photo-cathode produced using thealkali metal generating agent or the alkali metal generating deviceaccording to the present invention as described above, it is feasible toobtain the electron tube with satisfactory photoelectric conversioncharacteristics. In a case where the electron tube is provided with oneor more secondary-electron emitting surfaces (e.g., secondary-electronemitting surfaces of dynodes or the like), the secondary-electronemitting surfaces are also preferably produced using the alkali metalgenerating agent or the alkali metal generating device according to thepresent invention, from the aforementioned viewpoint.

An electron tube according to the present invention comprises at leastan electron multiplying part comprised of one or more dynodes eachhaving a secondary-electron emitting surface which emits secondaryelectrons corresponding to an incident electron. In this case, thesecondary-electron emitting surface according to the present inventioncan also be applied to the secondary-electron emitting surface in eachdynode.

When the electron tube comprises the secondary-electron emitting surfaceproduced using the alkali metal generating agent or the alkali metalgenerating device according to the present invention as described above,it is feasible to obtain the electron tube with satisfactoryphotoelectric conversion characteristics. In this case, thephoto-cathode provided in the aforementioned electron tube is alsopreferably produced using the alkali metal generating agent or thealkali metal generating device according to the present invention.

Furthermore, a method of production of a photo-cathode according to thepresent invention comprises a step of preparing the alkali metalgenerating agent or the alkali metal generating device according to thepresent invention, as a source of an alkali metal, a step of heating thealkali metal generating agent (in the case of the alkali metalgenerating device, a step of heating the alkali metal generating agenthoused in a case), and a step of guiding the alkali metal generated bythe heating of the alkali metal generating agent, to an area forformation of the photo-cathode. Through the above steps, we obtain thephoto-cathode comprising the alkali metal, which emits a photoelectroncorresponding to incident light.

In this manner, by using the alkali metal generating agent according tothe present invention, it is feasible to obtain the photo-cathode easyin formation and excellent in reproducibility of performance.

A method of production of a secondary-electron emitting surfaceaccording to the present invention comprises a step of preparing thealkali metal generating agent or the alkali metal generating deviceaccording to the present invention, as a source of an alkali metal, astep of heating the alkali metal generating agent (in the case of thealkali metal generating device, a step of heating the alkali metalgenerating agent housed in a case), and guiding the alkali metalgenerated by the heating of the alkali metal generating agent, to anarea for formation of the secondary-electron emitting surface. This canyield the secondary-electron emitting surface which emits secondaryelectrons corresponding to an incident electron.

In this manner, by using the alkali metal generating agent or the alkalimetal generating device according to the present invention, it isfeasible to obtain the secondary-electron emitting surface easy information and excellent in reproducibility of performance.

Furthermore, a method of production of an electron tube according to thepresent invention enables production of an electron tube having at leasta photo-cathode comprising an alkali metal, which emits a photoelectroncorresponding to incident light. Namely, the production method of theelectron tube comprises the steps of preparing the alkali metalgenerating agent or the alkali metal generating device according to thepresent invention, heating the alkali metal generating agent (in thecase of the alkali metal generating device, heating the alkali metalgenerating agent housed in a case), and guiding the alkali metalgenerated by the heating of the alkali metal generating agent, to anarea for formation of the photo-cathode.

When the photo-cathode is produced in this manner using the alkali metalgenerating agent or the alkali metal generating device according to thepresent invention, it is feasible to obtain the electron tube excellentin reproducibility of performance. In a case where the electron tubewith at least one secondary-electron emitting surface (e.g., asecondary-electron emitting surface of a dynode or the like) in additionto the photo-cathode is produced, the secondary-electron emittingsurface is also preferably produced using the alkali metal generatingagent or the alkali metal generating device according to the presentinvention, from the aforementioned viewpoint.

A method of production of an electron tube according to the presentinvention enables production of an electron tube having an electronmultiplying part comprised of one or more dynodes each having asecondary-electron emitting surface which emits secondary electronscorresponding to an incident electron. In this case, thesecondary-electron emitting surface in each dynode is produced bypreparing the alkali metal generating agent or the alkali metalgenerating device according to the present invention, heating the alkalimetal generating agent (in the case of the alkali metal generatingdevice, heating the alkali metal generating agent housed in a case), andguiding an alkali metal generated by the heating of the alkali metalgenerating agent, to an area for formation of the secondary-electronemitting surface.

When the secondary-electron emitting surface of the dynode is producedin this way using the alkali metal generating agent or the alkali metalgenerating device according to the present invention, it is feasible toobtain the electron tube excellent in reproducibility of performance. Inthis case, the photo-cathode in the electron tube is also preferablyproduced using the alkali metal generating agent or the alkali metalgenerating device according to the present invention, from theaforementioned viewpoint.

Each of embodiments of the present invention can be more fullyunderstood with the detailed description and accompanying drawings whichwill follow. It is noted that these embodiments are presented forillustrative purpose only but should not be construed as limiting theinvention.

A scope of further application of the present invention will becomeapparent from the detailed description below. However, the detaileddescription and specific incidences will present preferred embodimentsof the present invention but be given for purposes of illustration only,and it is apparent that various modifications and improvements withinthe spirit and scope of the present invention are obvious to thoseskilled in the art from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a configuration of an embodiment ofthe alkali metal generating agent according to the present invention;

FIG. 2 is a perspective view showing a configuration of a firstembodiment of the alkali metal generating device according to thepresent invention;

FIG. 3 is a sectional view along line I-I of the alkali metal generatingdevice (FIG. 2) in the first embodiment;

FIG. 4 is a sectional view showing a configuration of a secondembodiment of the alkali metal generating device according to thepresent invention;

FIG. 5 is a sectional view showing a configuration of a third embodimentof the alkali metal generating device according to the presentinvention;

FIG. 6 is a sectional view showing a configuration of a fourthembodiment of the alkali metal generating device according to thepresent invention;

FIG. 7 is a sectional view showing a configuration of a fifth embodimentof the alkali metal generating device according to the presentinvention;

FIG. 8 is an illustration showing a configuration of a photomultipliertube as a first embodiment of the electron tube according to the presentinvention;

FIG. 9 is an illustration for illustrating steps of production of thephoto-cathode and dynodes in the photomultiplier tube using the alkalimetal generating device shown in FIG. 6;

FIG. 10 is an illustration showing a configuration of a photomultipliertube as a second embodiment of the electron tube according to thepresent invention;

FIG. 11 is an illustration showing a configuration of a photo-tube as athird embodiment of the electron tube according to the presentinvention;

FIG. 12 is an illustration showing a configuration of an image tube(image intensifier) as a fourth embodiment of the electron tubeaccording to the present invention;

FIG. 13 is an illustration showing a configuration of a streak tube as afifth embodiment of the electron tube according to the presentinvention;

FIG. 14 is a table showing various characteristics (averages) of samplesof photomultiplier tubes produced using the alkali metal generatingagent according to the present invention, and a comparative example ofphotomultiplier tubes produced using a conventional alkali metalgenerating agent;

FIG. 15 is a table showing Life characteristics (%) of samples ofphotomultiplier tubes produced using the alkali metal generating agentaccording to the present invention, and a comparative example ofphotomultiplier tubes produced using a conventional alkali metalgenerating agent;

FIG. 16 is a graph showing radiant sensitivity characteristics andquantum efficiencies of samples of photomultiplier tubes produced usingthe alkali metal generating agent according to the present invention,and a comparative example of photomultiplier tubes produced using aconventional alkali metal generating agent; and

FIG. 17 is a graph showing relative outputs of Life characteristics of acomparative example of photomultiplier tubes produced using aconventional alkali metal generating agent, on the basis of the Lifecharacteristics of samples of photomultiplier tubes produced using thealkali metal generating agent according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Each of embodiments of the alkali metal generating agent and othersaccording to the present invention will be described below in detailwith reference to FIGS. 1 to 17. Identical or corresponding portionswill be denoted by the same reference symbols throughout the descriptionof the drawings, without redundant description.

(Alkali Metal Generating Agent)

FIG. 1 is a perspective view showing a configuration of a preferredembodiment of the alkali metal generating agent according to the presentinvention.

As described above, the alkali metal generating agent 1 shown in FIG. 1serves as a supply source for an alkali metal used in formation of thephoto-cathode or secondary-electron emitting surface. The alkali metalgenerating agent 1 in FIG. 1 is formed as a columnar pellet of all thecomponents by compression molding. This pellet form improveshandlability of the alkali metal generating agent 1 and facilitatesworks in a case where the agent is mounted in the after-described alkalimetal generating device, or works during production of thephoto-cathode, secondary-electron emitting surface, and electron tube.

An oxidizer in the above alkali metal generating agent 1 is comprised ofat least one vanadate with an alkali metal ion as a counter cation. Thisvanadate is preferably one expressed by chemical formula RVO₃. In thischemical formula R represents at least one metal element selected fromthe group consisting of Na, K, Rb, and Cs.

When the vanadate with a positive ion of the alkali metal elementrepresented by R in the above chemical formula, as a counter cation(hereinafter referred to as the vanadate) is used as an oxidizer, thealkali metal used as a material of the photo-cathodes in practical usecan be generated on a stable basis. A type of the oxidizer comprisingthe vanadate and a content of each component are appropriately selectedso as to match a component composition of the photo-cathode to beproduced or the secondary-electron emitting surface to be produced. Forexample, a combination of different types of materials may be includedat a predetermined content ratio, or only a single type may be included.

The reducer in the above alkali metal generating agent 1 initiates theredox reaction with the oxidizer at a predetermined temperature toreduce the alkali metal ion. There are no particular restrictions on thereducer as long as the alkali metal can be generated on a stable basis;however, the reducer is preferably at least one selected from the groupconsisting of Si, Zr, Ti, and Al. When these Si, Zr, Ti, and Al are usedeach singly or in an arbitrary combination as a reducer against theaforementioned oxidizer comprising the vanadate (e.g., when a mixture ofSi and Ti is used as a reducer), the alkali metal can be generated morestably.

A method of initiating the redox reaction between the reducer and theoxidizer is a method of heating the alkali metal generating agent to apredetermined temperature enough for the redox reaction to startproceeding, in an ambience adjusted in a predetermined vacuum. Here theterm “ambience adjusted in a predetermined vacuum” means an ambience inwhich a partial pressure of residual gas is 10⁻⁶-10⁻¹ Pa, preferably10⁻⁶-10⁻³ Pa.

The above alkali metal generating agent 1 may contain, for example, W,Al₂O₃, or the like as another component than the aforementioned oxidizerand reducer.

A production method of the above alkali metal generating agent 1 will bedescribed as an example below. The above alkali metal generating agent 1can be produced by a technique similar to that for the conventionalalkali metal generating agent using the chromate as an oxidizer, exceptfor use of the aforementioned vanadate as an oxidizer.

Namely, the first step is to select a vanadate as an oxidizer so as tomatch a component composition of the photo-cathode or thesecondary-electron emitting surface of dynode to be produced.

Subsequently, a measuring step, a crushing and mixing step, and aforming step are carried out in order. This measuring step is to measureappropriate amounts of the oxidizer and reducer (e.g., Si, Zr, Al, orthe like). The crushing and mixing step is to put these into a crusher(e.g., an agate mortar, a ball mill, or the like) and implement crushingand mixing simultaneously. In a case where the alkali metal generatingagent contains a component other than the oxidizer and reducer, thiscrushing and mixing step is arranged so that the component is puttogether with the oxidizer and reducer into the crusher to be mixed andcrushed, thereby obtaining powder of the alkali metal generating agent.The forming step is to press the resultant powder of the alkali metalgenerating agent by a powder presser to obtain the alkali metalgenerating agent 1 as a pellet formed in columnar shape.

In the above forming step, the alkali metal generating agent 1 is formedinto the columnar pellet by compression molding. However, where thealkali metal generating agent according to the present invention iscompressively formed, there are no particular restrictions on the shapethereof. The alkali metal generating agent according to the presentinvention can be compressively formed as in the above embodiment, butall the components thereof may be of the powder form. For example, thepowder before the forming as described above may be used as it is, orthe alkali metal generating agent may be formed once into pellet shapeand then crushed to be used as powder.

(Alkali Metal Generating Device)

Preferred embodiments of the alkali metal generating device according tothe present invention will be described below. FIG. 2 is a perspectiveview showing a configuration of a first embodiment of the alkali metalgenerating device according to the present invention. FIG. 3 is asectional view along line I-I of the alkali metal generating deviceshown in FIG. 2, which also shows a heating device.

The alkali metal generating device 2 shown in FIGS. 2 and 3 generates analkali metal to be used in formation of the photo-cathode or thesecondary-electron emitting surface. This alkali metal generating device2 has the alkali metal generating agent 1 shown in FIG. 1, and a metalcase 20 housing the alkali metal generating agent 1.

The case 20 is composed of a closed-end container 22 of a metal providedwith a recess for housing the pellet of the alkali metal generatingagent 1, and a lid member 24 of a metal welded to the closed-endcontainer 22 in a state in which it covers the entire recess of theclosed-end container 22. The recess of the closed-end container 22 has acapacity larger than the pellet of the alkali metal generating agent 1and is preferably formed in a shape similar to the pellet. Theclosed-end container 22 is provided with such an annular flange as tosurround the recess, and this flange is welded to the edge part of thelid member 24.

A non-welded portion to establish communication of the recess of theclosed-end container 22 (the space for housing the alkali metalgenerating agent 1) with the outside of the closed-end container 22 isprovided between the flange of the closed-end container 22 and the edgepart of the lid member 24, and this non-welded portion serves as adischarge port 23 for discharging the vapor of the alkali metalgenerated from the alkali metal generating agent 1, toward a portion forformation of the photo-cathode or toward a portion for formation of thesecondary-electron emitting surface of dynode.

A method of initiating the redox reaction of the alkali metal generatingagent 1 housed in this alkali metal generating device 2 can be a methodof heating the alkali metal generating agent 1 up to the predeterminedtemperature enough for the redox reaction to start proceeding, in theaforementioned ambience controlled in the predetermined vacuum.

More specifically, the alkali metal generating device preferably furthercomprises a heating device for generating the vapor of the alkali metal.There are no particular restrictions on this heating device as long asit has a configuration capable of heating the alkali metal generatingagent 1 in the aforementioned ambience. For example, the heating devicemay have a configuration based on a high-frequency heating method or aresistance heating method. From a viewpoint of readily and uniformlyheating the alkali metal generating agent 1, however, the heating devicepreferably has a configuration of heating the alkali metal generatingagent 1 by high-frequency heating.

The heating device of the high-frequency heating method, as shown inFIG. 3, has a high-frequency coil 25 wound around the case 20 housingthe alkali metal generating agent 1, and a high-frequency supply forsupplying a high-frequency current to the coil 25. For example, theheating device may be constructed in a configuration similar to that inthe case where the alkali metal generating agent containing theconventional chromate as an oxidizer is heated by the high-frequencyheating method. For example, a potential configuration is such that thealkali metal generating agent 1 is preliminarily mounted in an electrontube in which the photo-cathode and/or the secondary-electron emittingsurface of dynode is to be formed, it is heated by high-frequencyheating to generate the alkali metal vapor in the electron tube, and itis reacted to a predetermined portion where the photo-cathode and/or thesecondary-electron emitting surface of dynode is to be formed.

In the above production method of the alkali metal generating device 2,the alkali metal generating agent 1 is first produced as describedabove. Subsequently, the closed-end container 22 and lid member 24 arefabricated so as to match the shape and volume of this alkali metalgenerating agent 1. The closed-end container 22 is welded to the lidmember 24 in a state in which the alkali metal generating agent 1 ishoused in the recess. There are no particular restrictions on the methodof fabricating the closed-end container 22 and the lid member 24 and onthe method of welding the closed-end container 22 to the lid member 24,and the methods can be selected, for example, from the well-knowntechniques.

This alkali metal generating device 2 described was the one in which thealkali metal generating agent 1 formed in the pellet form was mounted,but another potential alkali metal generating device may be one in whichthe case 20 similar to the alkali metal generating device 2 is filledwith a powdered alkali metal generating agent before formation of thealkali metal generating agent 1, or with a powdered alkali metalgenerating agent obtained by crushing the alkali metal generating agent1.

A second embodiment of the alkali metal generating device according tothe present invention will be described next. FIG. 4 is a sectional viewshowing a configuration of the second embodiment of the alkali metalgenerating device according to the present invention, which also shows aheating device. The alkali metal generating device 3 shown in FIG. 4 iscomprised of a main body part 2A having a configuration similar to thealkali metal generating device 2 shown in FIGS. 2 and 3, a glass ampule32 containing the main body part 2A, and a bar-like support member 34coupled to the case 20 of the main body part 2A (having the dischargeport 23).

The glass ampule 32 has a tubular shape and, the inside diameter of thetop part (hereinafter referred to as a tip portion) opposed to a stembottom surface through which the support member 34 passes, is smallerthan those of the other portions. On the occasion of forming thephoto-cathode and/or the secondary-electron emitting surface of dynode,the alkali metal generating device 3 is coupled to an electron tube inwhich the photo-cathode and/or the secondary-electron emitting surfaceof dynode is to be formed. On that occasion, they are coupled so thatthe interior space in the glass ampule 32 is in communication with aspace of a portion where the photo-cathode and/or the secondary-electronemitting surface of dynode in the electron tube is to be formed. Namely,the glass ampule 32 is opened in formation of the photo-cathode and/orthe secondary-electron emitting surface.

One end of the support member 34 located in the glass ampule 32 iscoupled to the exterior surface of the lid member 24 of the case 20, andthe other end of the support member 34 projects through a through holeh32 provided in the glass ampule 32, to the outside of the ampule. Thissupport member 34 is in close fit to the interior surface of the throughhole h32 so that the interior of the ampule 32 is kept airtight.

For example, the heating device of the high-frequency heating method iscomprised of a high-frequency supply 26 capable of generating ahigh-frequency current, and a coil 25 (induction furnace) capable ofpassing the high-frequency current, which is coupled to thehigh-frequency supply 26. The coil 25 is arranged so as to surround themain body part 2A from the outside of the glass ampule 32 and can heatthe alkali metal generating device 3 to make the alkali metal generatingdevice 3 start generating the vapor of the alkali metal.

In a production method of the above alkali metal generating device 3,first, the alkali metal generating agent 1 is produced as describedabove, and the main body part 2A is produced by a method similar to thatof the alkali metal generating device 2. Subsequently, the supportmember 34 is welded to the main body part 2A, and thereafter the mainbody part 2A integrated with the support member 34 is sealed in theglass ampule 32. There are no particular restrictions on a method ofwelding the main body part 2A to the support member 34 and on a methodof sealing them in the glass ampule 32, and they can be selected, forexample, from the well-known techniques.

A third embodiment of the alkali metal generating device according tothe present invention will be described below. FIG. 5 is a sectionalview showing a configuration of the third embodiment of the alkali metalgenerating device according to the present invention, which also shows aheating device. The alkali metal generating device 4 shown in FIG. 5 iscomprised of a powdered or pelletized alkali metal generating agent 1A,and a metal (e.g., Ni) case 20A for housing the alkali metal generatingagent 1A. This alkali metal generating agent 1A has a compositionsimilar to that of the alkali metal generating agent 1 shown in FIG. 1.

This metal case 20A is made of a metal pipe provided with an interiorspace for housing the alkali metal generating agent 1. The edge portionsof apertures at the both ends of the case 20A are caulked, for example,by being beaten by a chisel or the like, so as to prevent the alkalimetal generating agent 1A from leaking out of the interior space.However, the caulked edge part of the case 20A is provided with a noncontact portion for establishing communication of the interior spacewith the outside of the case 20A, and this non contact portion serves asa discharge port 23 for discharging the vapor of the alkali metalgenerated from the alkali metal generating agent 1A, toward a portionfor formation of the photo-cathode or the secondary-electron emittingsurface. The size of this discharge port 23 is adjusted so as to preventthe alkali metal generating agent 1A from leaking out of the interiorspace.

In the case of this alkali metal generating device 4, the vapor of thealkali metal can be generated by heating the device in a manner similarto that in the aforementioned alkali metal generating devices 2 and 3.The heating device for heating this alkali metal generating device 4 iscomprised of a high-frequency coil 25 wound around the case 20, and ahigh-frequency supply 26 for supplying a high-frequency current to thecoil 25, as shown in FIG. 5.

In a production method of the above alkali metal generating device 4,first, the alkali metal generating agent 1A is produced as describedabove, and it is filled in the metal case (metal pipe) 20A.Subsequently, the apertures at the both ends of the metal case 20A arecaulked to obtain the alkali metal generating device 4. There are noparticular restrictions on a method of caulking the apertures at theboth ends of the metal case 20A, and it can be selected, for example,from the well-known techniques.

A fourth embodiment of the alkali metal generating device according tothe present invention will be described below. FIG. 6 is a sectionalview showing a configuration of the fourth embodiment of the alkalimetal generating device according to the present invention, which alsoshows a heating device. The alkali metal generating device 5 shown inFIG. 6 is comprised of a main body part 4A having a configurationsimilar to the alkali metal generating device 4 shown in FIG. 5, and aglass ampule 52 containing this main body part 4A. This glass ampule 52has a shape similar to the glass ampule 32 shown in FIG. 4. The insidediameter of the tip portion opposed to the bottom surface of the glassampule 52 is adjusted to a size enough for the main body part 4A to beintroduced into the interior.

On the occasion of forming the photo-cathode and/or thesecondary-electron emitting surface of dynode, this alkali metalgenerating device 5 is also coupled to an electron tube in which thephoto-cathode and/or the secondary-electron emitting surface of dynodeis to be formed, as in the case of the alkali metal generating device 3shown in FIG. 4. On that occasion, they are coupled so that the interiorspace in the glass ampule 52 is in communication with a space of aportion where the photo-cathode and/or the secondary-electron emittingsurface of dynode in the electron tube is to be formed.

In the case of this alkali metal generating device 5, the vapor of thealkali metal can also be generated by heating the device in a mannersimilar to that in the case of the aforementioned alkali metalgenerating devices 2 to 4. The heating device for heating this alkalimetal generating device 4 is comprised of a high-frequency coil 25 woundso as to surround the case 20, and a high-frequency supply 26 forsupplying a high-frequency current to the coil 25, as shown in FIG. 6.

In a production method of the above alkali metal generating device 5,first, the alkali metal generating agent 1A is produced as describedabove, and the main body part 4A is produced in the same manner as thealkali metal generating device 4. Subsequently, the main body part 4A issealed in the glass ampule 52. There are no particular restrictions on amethod of sealing the main body part 4A in the glass ampule 52, and itcan be selected, for example, from the well-known techniques.

A fifth embodiment of the alkali metal generating device according tothe present invention will be described below. FIG. 7 is a sectionalview showing a configuration of the fifth embodiment of the alkali metalgenerating device according to the present invention (including aheating device). The alkali metal generating device 6 shown in FIG. 7 ismainly comprised of a powdered or pelletized alkali metal generatingagent 1B, a metal case 20B housing the alkali metal generating agent 1A,two electrodes 64 placed at predetermined locations of this metal case20B, and an energizing device 68 electrically coupled to each of the twoelectrodes 64 and having a power supply for letting an electric currentflow from one electrode 64 to the other electrode 64.

This alkali metal generating agent 1B has a composition similar to thatof the alkali metal generating agent 1 shown in FIG. 1. The case 20B iscomposed of a metal pipe 62 having an interior space for housing thealkali metal generating agent 1, and two metal lid members 63 closingthe apertures at the both ends of the metal pipe 62. Each of the twoelectrodes 64 is coupled to either of the two metal lid members 63. Theenergizing device 68 is electrically coupled through a conductor 66 toeach of the two electrodes 64.

Furthermore, a discharge port 23 for establishing communication of theinterior space with the outside of the case 20B is provided in the sideface of the metal pipe 62. The alkali metal generating device is able todischarge the vapor of the alkali metal generated from the alkali metalgenerating agent 1A, through this discharge port 23 toward a portion forformation of the photo-cathode or the secondary-electron emittingsurface. The size of this discharge port 23 is adjusted so as to preventthe alkali metal generating agent 1B from leaking out of the interiorspace. There are no particular restrictions on the shape of thedischarge port 23 as long as it has the size as described above, and itmay be, for example, of slit shape.

In the case of this alkali metal generating device 6, it can heat thealkali metal generating agent 1B on the basis of the resistance heatingmethod by the energizing device 68. For example, when the electriccurrent of several amperes is fed to the metal case 20B, the alkalimetal generating agent 1B is heated by Joule heat generated in the metalcase 20B, whereby the vapor of the alkali metal can be generated.

In s production method of the above alkali metal generating device 6,the alkali metal generating agent 1B is first produced by a methodsimilar to that of the aforementioned alkali metal generating agent 1,and the alkali metal generating agent 1B is filled in the metal pipe 62.Subsequently, the both ends of the metal pipe 62 are closed each bywelding the lid member 63 so as to close the entire aperture.Furthermore, the electrodes 64 are coupled to the two lid members 63 andeach of the electrodes 64 is coupled to the energizing device 68,thereby obtaining the alkali metal generating device 6.

(Photo-Cathode, Secondary-Electron Emitting Surface, and Electron Tube)

Preferred embodiments of the photo-cathode, secondary-electron emittingsurface, and electron tube according to the present invention will bedescribed below.

First, a first embodiment of the electron tube according to the presentinvention will be described. FIG. 8 is an illustration showing aconfiguration of a photomultiplier tube as the first embodiment of theelectron tube according to the present invention. The photomultipliertube 7 shown in FIG. 8 has the configuration of a head-on typephotomultiplier tube having a transmissive photo-cathode (moreprecisely, in the case of the photomultiplier tube 7 shown in FIG. 8,the electron multiplying part is of a line focus type). Thisphotomultiplier tube 7 is mainly comprised of a photo-cathode C7; anelectron multiplying part D7 with dynodes D71-D79 havingsecondary-electron emitting surfaces FD7 to which photoelectrons e1emitted from the photo-cathode C7 are made to incident and which emitsecondary electrons e2 by making use of collision of the photoelectronse1; a focusing electrode E7 located between the photo-cathode C7 and theelectron multiplying part D72 and provided for focusing thephotoelectrons e1 emitted from the photo-cathode C7 and guiding thephotoelectrons e1 to the electron multiplying part D7; an anode A7 forcollecting multiplied secondary electrons e2 and extracting them as anelectric current to the outside; and a glass side tube 72 of a tubularshape (e.g., a cylindrical shape) (e.g., a Kovar glass, a UV glass, orthe like, or a metal material such as a Kovar metal, stainless steel, orthe like) for housing each of these electrodes; a voltage applying part(bleeder circuit) for regulation of potential is coupled to eachelectrode.

The photo-cathode C7 is mainly comprised of a substrate C71 (faceplate), and a layer C72 of a photoelectron emitting material (e.g., anintermetallic compound or a compound semiconductor) in a film form(hereinafter referred to as photoelectron emitting material layer C72)for emitting photoelectrons e1 corresponding to incident light L1.

This photo-cathode C7 is fixed to one aperture 72 a of the side tube 72.Namely, the substrate C71 that can transmit light to be utilized (e.g.,a glass substrate) is fusion-bonded and fixed to one aperture 72 a ofthe side tube 72 so that its light receiving surface FC71 is directed tothe outside. The photoelectron emitting material layer C72 is formed onthe internal surface (back surface) opposite to the light receivingsurface FC71 of the substrate C71.

The photoelectron emitting material layer C72 contains the alkali metalgenerated from either of the aforementioned alkali metal generatingagent and alkali metal generating device carrying it. Here thephotoelectron emitting material layer C72 is an intermetallic compound(compound semiconductor) containing the alkali metal as a constituentmaterial, or a compound semiconductor activated with the alkali metal.Specific examples of such materials include Sb—Cs, Sb—Rb—Cs, Sb—K—Cs,Sb—Na—K, Sb—Na—K—Cs, GaAs(Cs), InGaAs(Cs), InP/InGaAsP(Cs),InP/InGaAs(Cs), and so on. In the above examples, for example, (Cs) inGaAs(Cs) means that the material was obtained by an activation treatmentof GaAs with Cs. The same also applies to (Cs) in InP/InGaAsP(Cs) and inInP/InGaAs(Cs) hereinafter. Further examples include such photoelectronemitting materials as Cs—Te and Ag—O—Cs.

This photoelectron emitting material layer C72 is obtained by forming aconstituent material of a photoelectron emitting material to react withthe alkali metal, e.g. antimony or a compound semiconductor, on the backsurface of the substrate C71 and then reacting it with the vapor of thealkali metal.

A stem plate 78 of glass (e.g., a Kovar glass, a UV glass, or the like,or possibly a metal material such as a Kovar metal or stainless steel)is welded and fixed to the other aperture 72 b of the side tube 72. Inthis manner, a hermetic container is constructed of the side tube 72,the photo-cathode C7, and the stem plate 78.

Furthermore, an exhaust tube 73 is fixed to the center of the stem plate4. This exhaust tube 73 is used to evacuate the interior of the hermeticcontainer into a vacuum state by a vacuum pump, after completion ofassembly of the photomultiplier tube 7, and is also used as a guide tubefor guiding the alkali metal vapor into the hermetic container duringformation of the photoelectron emitting material layer C72.

The electron multiplying part D7 is provided with the first dynode D71to the ninth dynode D79 each having a plurality of plate-like dynodeelements. Each of the first dynode D71 to the ninth dynode D79 iscomprised of a substrate, and a layer of a secondary-electron emittingmaterial in film shape placed on the substrate and having asecondary-electron emitting surface FD7 to emit secondary electrons e2by making use of incident photoelectrons e1. In the descriptionhereinafter, the layer of the secondary-electron emitting material willbe referred to as a secondary-electron emitting material layer.

Each of the first dynode D71 to the ninth dynode D79 is supported in thehermetic container, for example, by a stem pin 75 (e.g., made of a Kovarmetal) provided so as to penetrate the hermetic container, and thedistal end of each stem pin 75 is electrically coupled to the firstdynode D71-the ninth dynode D79. The hermetic container is provided withpin holes for the respective stem pins 75 to pass, and, for example,each pin hole is filled with a tablet (e.g., made of a Kovar glass) usedas a hermetic seal. Each stem pin 75 is fixed through the tablet to thehermetic container. Furthermore, the stem pins 75 include the pins forthe first dynode D71-the ninth dynode D79 and a pin for the anode A7.

In this electron multiplying part D7, the secondary-electron emittingmaterial of the secondary-electron emitting material layer of eachdynode contains the alkali metal generated from either of theaforementioned alkali metal generating agent and alkali metal generatingdevice carrying it. There are no particular restrictions on thesecondary-electron emitting material in the secondary-electron emittingmaterial layer as long as it is a material containing the alkali metalas a constituent material or a material activated with the alkali metal.Specific examples of such materials include intermetallic compounds(compound semiconductors) of Sb with any one of the alkali metals.

Furthermore, the anode A7 fixed to the stem pin 75 is placed between theelectron multiplying part D7 and the stem plate 78. The focusingelectrode E7 is placed between the electron multiplying part D7 and thephoto-cathode C7. This focusing electrode E7 is provided with anaperture for discharging the focused stream of photoelectrons e1 towardthe electron multiplying part D7.

The other ends of the stem pins 75 coupled each to the first dynodeD71-the ninth dynode D79 and to the anode A7 are electrically coupled toa voltage applying part, whereby a predetermined voltage is supplied tothe first dynode D71-the ninth dynode D79 and to the anode A7. Theirpotentials are set so that the photo-cathode C7 has the same potentialas the focusing electrode E7 and so that the first dynode D71-the ninthdynode D79 and the anode A7 have their respective potentials decreasingin order from the top stage.

Accordingly, light L1 incident to the light receiving surface FC71 ofthe photo-cathode C7 is converted into photoelectrons e1 to be emittedfrom the internal surface FC72. Then the photoelectrons e1 are incidentto the electron multiplying part D7, to be multiplied in multiple stagesat the first dynode D71 to the ninth dynode D79, and electrons arefinally incident to the anode A7 to be outputted as an electric currentfrom the anode A7.

A method of production of the photomultiplier tube 7 (a preferredembodiment of the production method of the photo-cathode according tothe present invention, the production method of the secondary-electronemitting surface according to the present invention, and the productionmethod of the electron tube according to the present invention) will bedescribed below. There are no particular restrictions on the conditionsand procedure in the method of production of the photomultiplier tube 7except that the photo-cathode C7 and the first dynode D71 to the ninthdynode D79 are formed using the alkali metal generating agent or thealkali metal generating device according to the present invention. Thephotomultiplier tube 7 can be fabricated by the well-known techniques.

Namely, the side tube 72 is first integrated with the substrate C71 byheating (or a glass bulb in which the side tube and the substrate areintegrally formed may be used). In this stage, the photoelectronemitting material layer C72 is not formed yet on the substrate C71 ofthe photo-cathode C7 (or the substrate is in a state before alkaliactivation).

Subsequently, the anode A7, focusing electrode E7, and electronmultiplying part D7 are assembled on the lead pins 75 passing throughthe stem plate 78, and the assembly is inserted into the aperture 72 bof the side tube 72. In this stage, the secondary-electron emittingsurfaces are not formed yet on the substrates to become the dynodes inthe electron multiplying part D7 (or the substrates are in a statebefore alkali activation). Thereafter, the stem plate 78 is integratedwith the side tube 72 in the same manner as the substrate C71 was,thereby obtaining a hermetic container.

Described below is an example in which the photo-cathode C7 and thefirst dynode D71 to the ninth dynode D79 in the photomultiplier tube 7are formed using the alkali metal generating device 5 shown in FIG. 6.FIG. 9 is an illustration for explaining production steps of forming thephoto-cathode C7 and the first dynode D71 to the ninth dynode D79 in thephotomultiplier tube 7, using the alkali metal generating device 5 shownin FIG. 6. In FIG. 9, the detailed internal configuration of thephotomultiplier tube 7 is omitted.

First, the layer of the constituent material of the photoelectronemitting material layer C72 to react with the alkali metal ispreliminarily formed on the substrate C71, and the layer of theconstituent material of the secondary-electron emitting material layerto react with the alkali metal is preliminarily formed on the substratesof the respective dynodes D7. For example, an evaporation source (anevaporation source consisting of the constituent material of thephotoelectron emitting material layer C72 except for the alkali metal,or an evaporation source consisting of the constituent material of thesecondary-electron emitting material layers except for the alkali metal,such as Sb) is preliminarily mounted in the hermetic container.

Subsequently, the interior of the hermetic container is maintained in apredetermined vacuum state (in which the total pressure of residual gasinside the hermetic container is, for example, 10⁻⁶-10⁻³ Pa) by a vacuumpump. In this vacuum state the evaporation source is energized or heatedby high-frequency heating to evaporate an evaporative material formingthe evaporation source. Thereafter, the hermetic container is put intoan electric furnace or the like and maintained at a predeterminedtemperature to deposit the evaporative material on the substrate C71 oron the substrate of each dynode D7. It can also be contemplated that theevaporative material is preliminarily deposited on the substrate C71 oron the substrate of each dynode D7, using a separate evaporation system.

After the evaporation, an opening portion is formed in the exhaust tube73, whereupon the evaporative material inside the exhaust tube 73 isreleased to the outside. Next, as shown in FIG. 9, a closed-end glasstube 76 is prepared with the alkali metal generating device 5 beingplaced therein near the bottom part in a state in which the tip ofampule 52 is open, and an aperture of the glass tube 76 is coupled in anairtight state to the opening of the exhaust tube 73. The glass tube 76is provided with another aperture on its side face and this sideaperture is hermetically coupled to an aperture of glass tube 77 coupledto a vacuum pump. Thereafter, the vacuum pump is actuated to maintainthe interior of the hermetic container in a predetermined vacuum state(in which the total pressure of residual gas of the hermetic containeris, for example, 10⁻⁶-10⁻³ Pa) through the exhaust tube 73.

Then the alkali metal generating device 5 is heated by theaforementioned heating device of the high-frequency heating method toadvance the redox reaction of the oxidizer (vanadate) and the reducer ofthe alkali metal generating agent 1A in the alkali metal generatingdevice 5, thereby generating the vapor of the alkali metal.

At this time, the oxidizer with an alkali metal ion as a counter cation(vanadate) has the oxidizing power weaker than the chromate with analkali metal ion as a counter cation, so that the redox reaction withthe reducer proceeds moderately as compared with the case of thechromate. For this reason, the alkali metal vapor can be generated on astable basis, without rupture of the alkali metal generating agent 1Aitself or the case 20A housing it.

In other words, after the progress of oxidation reaction is onceinitiated by the heating device of the high-frequency heating method,the adjustment of reaction temperature can be readily accomplished byheating the, exhaust tube 73. Then the Cs vapor is guided to the distalportion of the glass ampule 52, so that the vapor of Cs or liquid of Csis collected at the distal part. Then the part of the hermetic containeris put into an electric furnace and the interior of the electric furnaceis maintained at a predetermined temperature (e.g., 200° C.). On thatoccasion, the alkali metal generating device 5 is moved toward thehermetic container to insert the distal portion of the ampule 52 of thealkali metal generating device 5 into the hermetic container.

This permits the distal portion of the ampule 52 to be kept at thepredetermined temperature in the electric furnace, whereby the vapor ofthe alkali metal such as Cs can be stably discharged from the distalportion. Namely, the photo-cathode C7 and the first dynode D71 to theninth dynode D79 with performance comparable to that of thephoto-cathode and dynodes produced using the conventional chromate canbe produced without any difficulty and with good reproducibility.

The vapor of the alkali metal such as Cs stably discharged from thedistal portion of the glass ampule 52 into the hermetic container inthis way reacts with the layer of the preliminary form for reacting withthe alkali metal of the photo-cathode C7 to form the photoelectronemitting material layer C72 or with the layers of the preliminary formfor reacting with the alkali metal of the first dynode D71 to the ninthdynode D79 to form the secondary-electron emitting material layers, soas to make the photoelectron emitting material or the secondary-electronemitting material. Then the photoelectron emitting material layer C72with satisfactory spectral response characteristics or thesecondary-electron emitting surfaces FD7 with satisfactory amplificationefficiency are formed.

Then the distal end of the alkali metal generating device 5 is taken outof the hermetic container and the alkali metal generating device 5 ismoved to the bottom side of the glass tube 76. Thereafter, the glasstube 76 is cut off from the exhaust tube 73.

The above operations are repeated for every alkali metal generatingagent used, to form the photoelectron emitting material layer C72 in apredetermined chemical composition on the substrate C71 and form thesecondary-electron emitting material layers in a predetermined chemicalcomposition on the substrates of the dynodes. After use of the lastalkali metal generating device 5, the vacuum pump is actuated in a statein which the interior of the photomultiplier tube 7 is maintained at apredetermined temperature, whereby the residual gas in thephotomultiplier tube 7 is thoroughly removed, so as to eliminate thealkali metal or the gas evolving from the other evaporation sources,which remains physically adsorbed at portions other than thephotoelectron emitting material or the secondary-electron emittingmaterial in the photomultiplier tube 7. Thereafter, the opening part ofthe exhaust tube 73 in the hermetic container is sealed to obtain thephotomultiplier tube 7 with satisfactory photoelectric conversioncharacteristics.

A second embodiment of the electron tube according to the presentinvention will be described below. FIG. 10 is an illustration showing aconfiguration of a photomultiplier tube as the second embodiment of theelectron tube according to the present invention. This FIG. 10 showsanother configuration of the photomultiplier tube 7 shown in FIG. 8.

The photomultiplier tube 7A shown in FIG. 10 is mainly comprised of anelectrode part 71, an alkali metal generating device 2 fixed to theelectrode part 71, a glass container with a nearly cylindrical contourhousing the electrode part 71 and the alkali metal generating device 2,and stem pins 75A electrically coupled to respective electrodes of theelectrode part 71. The glass container is comprised of a glass side tube72A and a glass stem plate 78A. The electrode part 71 is constructed ofan electron multiplying part consisting of a photo-cathode, a focusingelectrode, and a plurality of dynodes, and an anode, as in the case ofthe photomultiplier tube 7 of FIG. 8. Each stem pin 75A is coupled to avoltage applying part, as in the photomultiplier tube 7 of FIG. 8.

The alkali metal generating device 2 has a configuration similar to thealkali metal generating device shown in FIGS. 2 and 3. The alkali metalgenerating device 2 is used in formation of the photo-cathode and thedynodes of the electron multiplying part in the electrode part 71. Thisalkali metal generating device 2 is fixed to the electrode part 71 bymetal wire. FIG. 10 shows the configuration in which there is one alkalimetal generating device 2, but it is also possible to adopt aconfiguration wherein a plurality of alkali metal generating devices 2with their respective alkali metal generating agents 1 having differentchemical compositions according to the chemical composition of thephoto-cathode to be formed or according to the chemical composition ofthe secondary-electron emitting surfaces of the dynodes are fixed to theelectrode part 71.

This photomultiplier tube 7A is a side-on type photomultiplier tubehaving a reflective photo-cathode formed on a metal substrate. For thatpurpose, the columnar side tube 72A forming the glass container isoptically transparent to light to be used, and the substrate of thephoto-cathode placed in the electrode part 71 is a substrate of a metalsuch as Ni, for example. This photomultiplier tube 7A has theconfiguration, for example, similar to the known side-on typephotomultiplier tube, except for the electrode part 71 and alkali metalgenerating device 2 fixed to the electrode part 71.

In a production method of the above photomultiplier tube 7A, first, theglass stem plate 78A having the lead pins 75A and the electrode part 71fixed to the lead pins 75A is fixed to an aperture of tubular glass sidetube 72A with one bottom surface closed. On that occasion, the alkalimetal generating device 2 is also attached to the electrode part 71. Anexhaust tube 73A coupled to the stem plate 78A is once opened, and theopening part thereof is coupled to a suction port of a vacuum pump.

In this case, a layer (e.g., an antimony layer) for reacting with thealkali metal to form an intermetallic compound is preliminarily formedon the photo-cathode forming substrate and on the secondary-electronemitting surfaces of dynodes.

In either of the above cases, the interior of the glass container ismaintained in a predetermined vacuum state by the vacuum pump. In thisvacuum state the heating device of the aforementioned high-frequencyheating method heats the alkali metal generating device 2 or evaporationsource from the outside of the glass container. This results in formingthe photoelectron emitting material layer of the photo-cathode and thesecondary-electron emitting material layers of the dynodes.

In the case of this photomultiplier tube 7A, when the alkali metalgenerating device 2 is heated by the heating device of thehigh-frequency heating method, the oxidizer (vanadate) with an alkalimetal ion as a counter cation also moderately advances the redoxreaction with the reducer, as compared with the case of the chromate.For this reason, the alkali metal vapor can be generated on a stablebasis, without rupture of the alkali metal generating agent 1 itself orthe case 20 housing it. The appearance will not be spoiled even if thecase 20 is left in the glass container.

After the progress of oxidation reaction is once initiated by theheating device of the high-frequency heating method, the glass containeris put into an electric furnace kept at a predetermined temperature andis subjected to temperature control, whereby the vapor of the alkalimetal can be made stably to react with the photo-cathode forming portionor the portions for formation of the secondary-electron emittingsurfaces. The alkali metal vapor reacts with the layer of thepreliminary form for reacting with the alkali metal of the photo-cathodeto form the photoelectron emitting material layer or with the layers ofthe preliminary form for reacting with the alkali metal of the dynodesto form the secondary-electron emitting material layers, thereby makingthe photoelectron emitting material or the secondary-electron emittingmaterial. Then the photo-cathode with satisfactory spectral responsecharacteristics or the secondary-electron emitting surfaces withsatisfactory multiplication efficiency are formed.

After the formation of the photo-cathode or the secondary-electronemitting surfaces, the vacuum pump is actuated in a state in which theinterior of the photomultiplier tube 7A is maintained at a predeterminedtemperature, whereby the residual gas in the photomultiplier tube 7A isthoroughly removed. This results in eliminating the alkali metal or thegas evolving from the other evaporation sources, which remainsphysically adsorbed at the portions other than the photoelectronemitting material or the secondary-electron emitting surfaces in thephotomultiplier tube 7. Thereafter, the opening part of the exhaust tube73A of the glass container is sealed to obtain the photomultiplier tube7A with satisfactory photoelectric conversion characteristics.

In the formation of this photomultiplier tube 7A, the alkali metalgenerating device 3 shown in FIG. 4 or the alkali metal generatingdevice 5 shown in FIG. 6 may be used instead of the alkali metalgenerating device 2. In this case, the photomultiplier tube 7A is alsoproduced according to procedure similar to that of the aforementionedphotomultiplier tube 7.

The above described the various electron tubes with the photomultipliertube, as the electron tubes according to the present invention, but theelectron tube according to the present invention, in the case having theconfiguration of the photomultiplier tube, may be any electron tube ifat least one of the photoelectron emitting material layer of thephoto-cathode and the secondary-electron emitting material layers of thedynodes is formed using the vapor of the alkali metal generated from thealkali metal generating agent or the alkali metal generating devicecarrying it according to the present invention. For example, as in theabove embodiments (the photomultiplier tube 7 and the photomultipliertube 7A), the photo-cathode and the dynodes both may be formed using thevapor of the alkali metal generated from the alkali metal generatingagent or the alkali metal generating device carrying it according to thepresent invention. Only either of the photoelectron emitting materiallayer of the photo-cathode and the secondary-electron emitting materiallayers of the dynodes may be formed using the vapor of the alkali metalgenerated from the alkali metal generating agent or the alkali metalgenerating device carrying it according to the present invention. It is,however, preferable to adopt the former in terms of productionefficiency.

In the electron tube according to the present invention, where it hasthe configuration with dynodes, as in the above embodiments(photomultiplier tube 7 and photomultiplier tube 7A), there are noparticular restrictions on the shape of the dynodes. For example, theabove embodiments described the examples wherein the line focus typedynodes were mounted as dynodes D7, but the electron tube may beprovided with dynodes of the box type, the Venetian blind type, the meshtype, the metal channel dynode type, and so on.

A third embodiment of the electron tube according to the presentinvention will be described below. FIG. 11 is an illustration showing aconfiguration of a photo-tube as the third embodiment of the electrontube according to the present invention.

The photo-tube 8 shown in FIG. 11 has a configuration similar to thephotomultiplier tube 7, except that the photo-tube 8 does not have thefocusing electrode E7 and the electron multiplying part D7 forming thephotomultiplier tube 7 shown in FIG. 8. The photo-cathode C7 of thisphoto-tube 8 can also be readily produced in the same manner as thephoto-cathode C7 of the aforementioned photomultiplier tubes 7 and 7A.Then satisfactory photoelectric conversion characteristics are achievedas to the resultant photo-tube 8. A glass container of this electrontube 8 is comprised of a glass side tube 72, a photo-cathode C7, and aglass stem plate 78.

A fourth embodiment of the electron tube according to the presentinvention will be described below. FIG. 12 is an illustration showing aconfiguration of an image tube (image intensifier) as the fourthembodiment of the electron tube according to the present invention.

The image intensifier 9 shown in FIG. 12 is provided with aphoto-cathode C7, a micro-channel plate MCP for multiplyingphotoelectrons e1 emitted from the photo-cathode C7, and a fluorescentscreen 90 for converting electrons e2 emitted from the micro-channelplate MCP, into light. An exhaust tube is provided in the side tube 72.The MCP is not subjected to alkali activation with the alkali metalgenerating agent. The image tube 9 may be constructed in a configurationwithout MCP. The above image tube also embraces an X-ray image tube forconverting an X-ray image into a visible image.

In the case of the intensifier 9 shown in FIG. 12, the photo-cathode C7is configured so that a photoelectron emitting material layer C72 (e.g.,a photo-cathode in a composition of GaAs—CsO or the like) performsphotoelectric conversion of incident light L1 carrying opticaltwo-dimensional information and so that an inside surface FC72 emitsphotoelectrons e1 corresponding to the incident light L1. Then themicro-channel plate MCP is maintained at a high potential relative tothe photo-cathode C7 by voltage applying part 74 and, upon incidence ofphotoelectrons e1, the micro-channel plate emits secondary electrons e2by making use of collision of the photoelectrons e1. For example, thevoltage of about 1000 V is placed between entrance surface F91 forphotoelectrons e1 and secondary-electron exit surface F92 in themicro-channel plate MCP by a predetermined voltage applying part, so asto achieve an electron multiplication rate of several thousand toseveral ten thousand times.

The fluorescent screen 90 is comprised of a transparent substrate 94, aphosphor layer 92 formed on the transparent substrate 94, and anelectrode 75 formed on the surface of the phosphor layer 92. Thiselectrode 75 is an electrode for accelerating multiplied secondaryelectrons e2, and is regulated at a predetermined potential to apply avoltage. Namely, this electrode 75 is also maintained at a highpotential relative to the secondary-electron exit surface F92 andvoltage applying part 74 of the micro-channel plate MCP.

Furthermore, there are no particular restrictions on constituentmaterials forming the phosphor layer 92 and on constituent materialsforming the substrate 94, and the well-known materials can be used forthem. For example, it is also possible to employ a configuration inwhich an optical fiber plate constructed of a bundle of optical fibersis used as the substrate 94 and in which a thin film of metal is placedbetween the optical fiber plate and the phosphor layer.

The photo-cathode C7 of this image intensifier 9 can also be readilyproduced in the same manner as the photo-cathode C7 of theaforementioned photomultiplier tubes 7 and 7A. Then satisfactoryphotoelectric conversion characteristics are achieved as to theresultant image intensifier 9.

A fifth embodiment of the electron tube according to the presentinvention will be described. FIG. 13 is an illustration showing aconfiguration of a streak tube as the fifth embodiment of the electrontube according to the present invention.

In the streak tube 10 shown in FIG. 13, just as in the photomultipliertube 7 shown in FIG. 8, the photo-cathode C7 is placed on the side ofone aperture 72 a of side tube 72. Measured light L1 incident from theoutside is converted into photoelectrons in the photoelectron emittingmaterial layer C72 of this photo-cathode C7.

An accelerating electrode 11 of flat plate shape for accelerating thephotoelectrons emitted from the inner surface FC72 is placed next to thephoto-cathode C7 in the side tube 72. This accelerating electrode 11 isplaced so that a normal to the electrode surface is nearly parallel to anormal to the inner surface FC72. A focusing electrode 12 for focusingthe primary electrons accelerated by the accelerating electrode 11 isplaced next to the accelerating electrode 11. The focusing electrode 12is comprised of a pair of electrodes of flat plate shape, and is placedso that electrode surfaces of the respective electrodes are parallel toeach other and nearly perpendicular to the inner surface FC72. An anodeA10 of disk shape is placed next to the focusing electrode 12, is formedin a configuration with a through hole H10 for the primary electronsfocused by the focusing electrode 12 to pass, and is arranged toelectrically attract the electrons so as to guide them through thethrough hole H10.

Furthermore, a deflecting electrode 14 for sweeping the electronspassing through the aperture H10 of anode A10 at high speed is placednext to the anode A10. This deflecting electrode 14 is comprised of apair of electrodes of flat plate shape located as opposed to each other.Normals to electrode surfaces in this pair of electrodes are parallel toeach other and each of the normals is perpendicular to the normal to theinner surface FC72. A predetermined deflection voltage is appliedbetween the pair of electrodes of flat plate shape, whereby the primaryelectrons emitted through the aperture H10 from the anode A10 are sweptin a predetermined direction.

A micro-channel plate MCP for multiplying the electrons swept by thedeflecting electrode 14 is placed next to the deflecting electrode 14.It is noted that this streak tube 10 may be constructed in aconfiguration without this micro-channel plate MCP.

A fluorescent screen 90 for converting electrons emitted from themicro-channel plate MCP, into light is placed next to the micro-channelplate MCP. This fluorescent screen 90 has a configuration similar to thefluorescent surface 90 shown in FIG. 12. A hermetic container isconstructed of the face plate C71, transparent substrate 94, and sidetube 72.

In the above-described streak tube 10, when measured light L1 is guidedthrough a slit plate to the photo-cathode C7, this measured light isconverted into an electron image and the electron image is acceleratedby the accelerating electrode 11 to be attracted to the anode A10. Thenthis electron image passes the anode A10 to go into between the twodeflecting electrodes 14, where electrons are swept at high speed in adirection parallel to the normal direction to the electrode surfaces ofthe deflecting electrode 14. The reason why electrons are swept at highspeed is that the number of electrons passing the deflecting electrode14 varies corresponding to temporal change of optical intensity of themeasured light varying at high speed against time.

The electrons thus swept at high speed are multiplied by themicro-channel plate MCP, and electrons multiplied by the micro-channelplate MCP are converted into an optical image (also called a streakimage) at the fluorescent screen 90. In this way, a temporal change ofintensity of the measured light is converted to a spatial change ofintensity at the fluorescent screen 90. Since in the operation of thestreak tube the electrons are swept in synchronism with their passingtime, the temporal change can be determined by analyzing the spatialchange of optical intensity, i.e., the streak image projected on thephosphor electrode 90.

The photo-cathode of this streak tube 10 can also be readily produced inthe same manner as in the aforementioned photomultiplier tubes 7 and 7A.Then satisfactory photoelectric conversion characteristics can beachieved as to the resultant streak tube 10.

(Experiments)

The present invention will be further described below in more detailwith samples and a comparative example of the alkali metal generatingagent according to the present invention. It is, however, noted that thepresent invention is by no means intended to be limited to the samplesof examples below.

(Samples)

The Inventors fabricated as samples, a plurality of photomultipliertubes (having a configuration similar to FIG. 10) in a configurationsimilar to the commercially available side-on type photomultipliertubes, except that the photo-cathode mounted was one formed using thealkali metal generating agent described below (antimony alkaliphoto-cathode: Sb—Cs, substrate material: Ni) and the secondary-electronemitting surfaces mounted were those formed using the alkali metalgenerating agent described below (Cs—Sb).

The alkali metal generating agent for formation of the photo-cathode wasone containing a vanadate (CsVO₃) as an oxidizer and Si as a reducer,and having the total weight of 94 mg. The shape of samples of the alkalimetal generating agent was the pellet shape similar to FIG. 1, and asubstance amount ratio was CsVO₃:Si=1:1.1.

In the samples the secondary-electron emitting surfaces were also formedusing the alkali metal generating agent for formation of thephoto-cathode.

The alkali metal generating agent was prepared by successively carryingout the aforementioned measuring step, crushing and mixing step, andforming step with the mixture of the above vanadate.

Then each of these samples of the alkali metal generating agent washoused in the metal case 20 shown in FIGS. 2 and 3, and this metal case20 was housed in the glass ampule 32 as shown in FIG. 4, therebyfabricating the alkali metal generating device of the configurationsimilar to the alkali metal generating device 3.

The photo-cathode and secondary-electron emitting surfaces werefabricated by a method similar to the production method of thephotomultiplier tube 7 described with FIG. 9, except for the use of thealkali metal generating device, to obtain photomultiplier tubes.

(Comparative Example)

On the other hand, the Inventors also prepared as a comparative example,a plurality of photomultiplier tubes having a configuration similar tothe commercially available side-on type photomultiplier tubes, by amethod similar to that of the above samples. The photo-cathode mountedin the photomultiplier tubes in this comparative example was aphoto-cathode formed by using the conventional alkali metal generatingagent containing a chromate (Cs₂CrO₄) as an oxidizer and Si as a reducer(antimony alkali photo-cathode: Sb—Cs). The total weight of the alkalimetal generating agent in the comparative example was 82 mg, and asubstance amount ratio thereof was Cs₂CrO₄:Si=1:1.3.

(Characteristic Evaluation Tests)

The photomultiplier tubes of the samples and comparative exampleprepared as described above were evaluated by measuring variouscharacteristics of cathode output (Sk:_(″)A/1 m), anode output(Sp:_(″)A/1 m), dark current (Idb:nA), and After Pulse (%) and alsomeasuring radiant sensitivity (mA/W) and Life (%) (secular change ofSp). FIG. 14 to FIG. 17 are tables and graphs showing the measurementresults thereof. The measurements of the above various characteristicswere carried out based on the methods described in “PhotomultiplierTubes—Their fundamentals and applications—,” (Editorial Board ofHamamatsu Photonics K.K.) (e.g., p 34-p 39: “Fundamental characteristicsof photo-cathode,” p 60-p 73: “Various characteristics ofphotomultiplier tube,” and so on).

FIG. 14 is a table showing the various characteristics (averages) in thesamples of the photomultiplier tubes fabricated using the alkali metalgenerating agent according to the present invention and in thecomparative example of the photomultiplier tubes fabricated using theconventional alkali metal generating agent. FIG. 15 is a table showingLife characteristics (%) in the samples of the photomultiplier tubesfabricated using the alkali metal generating agent according to thepresent invention and in the comparative example of the photomultipliertubes fabricated using the conventional alkali metal generating agent.FIG. 16 is a graph showing the radiant sensitivity characteristics andquantum efficiencies in the samples of the photomultiplier tubesfabricated using the alkali metal generating agent according to thepresent invention and in the comparative example of the photomultipliertubes fabricated using the conventional alkali metal generating agent.In this FIG. 16, graph G1610 indicates the radiant sensitivity of thephotomultiplier tubes of the samples, graph G1620 the radiantsensitivity of the photomultiplier tubes of the comparative example,graph G1630 the quantum efficiency of the photomultiplier tubes of thesamples, and graph G1640 the quantum efficiency of the photomultipliertubes of the comparative example. FIG. 17 is a graph showing relativeoutputs of Life characteristics in the comparative example of thephotomultiplier tubes fabricated using the conventional alkali metalgenerating agent, on the basis of the Life characteristics of thesamples of the photomultiplier tubes fabricated using the alkali metalgenerating agent according to the present invention. In this FIG. 17,graph P1 indicates the Life characteristics of the photomultiplier tubesof the samples, and graphs P2-P4 indicate relative outputs of Lifecharacteristics of the photomultiplier tubes of the comparative example,on the basis of the graph P1.

Particularly, the relative outputs P2-P4 of Life characteristics of thephotomultiplier tubes of the comparative example (commercially availablephotomultiplier tubes) in FIG. 17 were obtained by measuring data of aplurality of (35) samples. Namely, graph P2 indicates the average of alldata, graph P3 the average of all data+_(″) (_(″) is a standarddeviation), and graph P4 the average of all data−_(″) (_(″) is astandard deviation).

As apparent from the measurement results shown in FIG. 16, it wasconfirmed that the samples fabricated as the photomultiplier tubes ofthe present invention had the radiant sensitivity and quantum efficiencyequivalent to those of the conventional photomultiplier tubes of thecomparative example.

As also seen from the table shown in FIG. 15, it was confirmed that thephotomultiplier tubes of the samples had the Life characteristicsequivalent to those of the conventional photomultiplier tubes of thecomparative example. This Life characteristic evaluation test wasconducted under the following conditions: the operating current (outputcurrent) of each photomultiplier tube was 100 _(″) Å and the appliedvoltage between the photo-cathode and the anode was 1000 V. The valuesof Life characteristics (relative outputs) in the table shown in FIG. 15indicate relative values relative to 100% at the value of the anodeoutput (Sp) after a lapse of one hour since a start of measurement.

Furthermore, as shown in FIG. 17, it was confirmed that the samples ofthe photomultiplier tubes according to the present invention (graph P1indicating the Life characteristics of the samples in FIG. 17 representsan average of five samples) yielded the relative outputs indicating Lifecharacteristics approximately equivalent to those of the photomultipliertubes of the comparative example and had excellent reproducibility ofcharacteristics.

As shown in the table of FIG. 14, it was confirmed that thephotomultiplier tubes of the samples (the photomultiplier tubesaccording to the present invention) had the cathode output, anodeoutput, dark current, and After Pulse characteristics equivalent tothose of the conventional photomultiplier tubes of the comparativeexample. The measurement results of After Pulse characteristics wereobtained by using an LED (semiconductor laser) to make eachphotomultiplier tube in the samples and comparative example output apulse signal and performing the calculation based on After Pulsegenerated between 0.5 and 10_(″) sec after the output of the signal.

It is apparent that the present invention can be modified in variousways, from the above description of the present invention. Suchmodifications are to be understood not to depart from the spirit andscope of the present invention, and all improvements obvious to thoseskilled in the art should be included in the scope of claims which willfollow.

INDUSTRIAL APPLICABILITY

As described above, the present invention enables the reaction rate tobe readily controlled by controlling only the reaction temperature inthe redox reaction between the oxidizer with an alkali metal ion as acounter cation (vanadate) and the reducer. For this reason, it becomesfeasible to provide the alkali metal generating agent for formation ofthe photo-cathode or the secondary-electron emitting surface capable ofstably generating the alkali metal at predetermined temperature. It alsobecomes feasible to provide the alkali metal generating device providedwith the alkali metal generating agent and thereby permitting easycontrol of the alkali metal generating rate.

By using the alkali metal generating agent or the alkali metalgenerating device according to the present invention, it is feasible toobtain the photo-cathode with satisfactory spectral responsecharacteristics, the secondary-electron emitting surface withsatisfactory multiplication efficiency, and the electron tube withsatisfactory optical characteristics and electrical characteristics.

Furthermore, by using the alkali metal generating agent or the alkalimetal generating device according to the present invention, it becomesfeasible to provide the production method of the photo-cathode, theproduction method of the secondary-electron emitting surface, and theproduction method of the electron tube easy in formation and excellentin reproducibility of resultant performance.

1. An alkali metal generating agent as a supply source of an alkalimetal used in formation of a photo-cathode for emitting a photoelectroncorresponding to incident light or a secondary-electron emitting surfacefor emitting secondary electrons corresponding to an incident electron,said alkali metal generating agent comprising: an oxidizer comprising atleast one vanadate with an alkali metal ion as a counter cation; and areducer for initiating a redox reaction with the oxidizer at apredetermined temperature to reduce the alkali metal ion.
 2. An alkalimetal generating agent according to claim 1, wherein the vanadate isexpressed by a chemical formula RVO₃, where R is at least one metalelement selected from the group consisting of Na, K, Rb, and Cs.
 3. Analkali metal generating agent according to claim 1, wherein the reduceris at least one selected from the group consisting of Si, Zr, Ti, andAl.
 4. An alkali metal generating agent according to claim 1, the alkalimetal generating agent being of a powder form.
 5. An alkali metalgenerating agent according to claim 1, the alkali metal generating agentbeing formed in a pellet form having a predetermined shape bycompression molding.
 6. An alkali metal generating device for generatingan alkali metal used in formation of a photo-cathode for emitting aphotoelectron corresponding to incident light or a secondary-electronemitting surface for emitting secondary electrons corresponding to anincident electron, said alkali metal generating device comprising: acase; a supply source housed in the case and comprising an alkali metalgenerating agent according to claim 1; and a discharge port provided inthe case and adapted for discharging a vapor of the alkali metalgenerated in the supply source, from an interior space of the casehousing the supply source, toward the exterior of the case.
 7. An alkalimetal generating device according to claim 6, wherein the case is madeof a metal.
 8. An alkali metal generating device according to claim 6,wherein the case comprises: a hollow container of a metal havingapertures at both ends and provided with the discharge port in a sideface thereof; and lid members of a metal covering the respectiveapertures at the both ends of the hollow container.
 9. An alkali metalgenerating device according to claim 6, wherein the case is a hollowcontainer of a metal having apertures at both ends thereof, wherein theapertures at the both ends of the hollow container are hermeticallyclosed in a state in which the hollow container secures an interiorspace for housing the alkali metal generating agent, and wherein thedischarge port is provided in at least one of the both ends of thehollow container hermetically closed.
 10. An alkali metal generatingdevice according to claim 6, wherein the alkali metal generating agentis formed in a pellet form having a predetermined shape, wherein thecase is comprised of a closed-end container of a metal having a recessfor housing the alkali metal generating agent, and a lid member of ametal welded to the closed-end container in a state in which the lidmember covers an aperture of the recess, and wherein the discharge portof the case is formed in a non-welded portion between the closed-endcontainer and the lid member.
 11. An alkali metal generating deviceaccording to claim 6, further comprising a glass ampule housing theentire case.
 12. An alkali metal generating device according to claim 6,further comprising a heating device for initiating the redox reaction ofthe alkali metal generating agent to generate the vapor of the alkalimetal.
 13. An alkali metal generating device according to claim 12,wherein the heating device comprises a high-frequency supply for heatingthe alkali metal generating agent by high-frequency heating.
 14. Aphoto-cathode for emitting a photoelectron corresponding to incidentlight, said photo-cathode comprising the alkali metal generated from analkali metal generating agent according to claim
 1. 15. A photo-cathodefor emitting a photoelectron corresponding to incident light, saidphoto-cathode comprising the alkali metal generated from an alkali metalgenerating device according to claim
 6. 16. A secondary-electronemitting surface for emitting secondary electrons corresponding to anincident electron, said secondary-electron emitting surface comprisingthe alkali metal generated from an alkali metal generating agentaccording to claim
 1. 17. A secondary-electron emitting surface foremitting secondary electrons corresponding to an incident electron, saidsecondary-electron emitting surface comprising the alkali metalgenerated from an alkali metal generating device according to claim 6.18. An electron tube comprising a photo-cathode according to claim 14.19. An electron tube according to claim 18, further comprising: anelectron multiplying part comprised of one or more dynodes each having asecondary-electron emitting surface for emitting secondary electrons inaccordance with incidence of the photoelectron emitted from thephoto-cathode; and an anode for collecting the secondary electronsoutputted from the electron multiplying part and extracting thecollected secondary electrons as an electric current to the outside. 20.An electron tube according to claim 18, further comprising: an anode forcollecting the photoelectron emitted from the photo-cathode andextracting the collected photoelectron as an electric current to theoutside.
 21. An electron tube according to claim 18, said electron tubecomprising an image tube having at least a fluorescent screen forconverting the photoelectron emitted from the photo-cathode, into light.22. An electron tube according to claim 18, further comprising a streaktube comprising: an accelerating electrode for accelerating thephotoelectron emitted from the photo-cathode; a focusing electrode forfocusing the photoelectron accelerated by the accelerating electrode; ananode having an aperture through which the photoelectron focused by thefocusing electrode can pass; a deflecting electrode having a pair ofelectrode plates opposed to each other and adapted to be able to sweepthe photoelectron having passed through the aperture provided in theanode, in a predetermined direction by a predetermined deflectionvoltage applied between the pair of electrode plates; and a fluorescentscreen for converting the photoelectron deflected by the deflectingelectrode, into light.
 23. An electron tube comprising an electronmultiplying part comprised of one or more dynodes each having asecondary-electron emitting surface according to claim
 16. 24. Anelectron tube according to claim 23, further comprising: a photo-cathodefor emitting a photoelectron corresponding to incident light, toward theelectron multiplying part; and an anode for collecting secondaryelectrons emitted from the electron multiplying part and extracting thecollected secondary electrons as an electric current to the outside. 25.A method of production of a photo-cathode comprising an alkali metal foremitting a photoelectron corresponding to incident light, said methodcomprising the steps of: preparing an alkali metal generating agentaccording to claim 1, as a source of the alkali metal; heating thealkali metal generating agent; and guiding the alkali metal generated bythe heating of the alkali metal generating agent, to an area forformation of the photo-cathode.
 26. A method of production of aphoto-cathode comprising an alkali metal for emitting a photoelectroncorresponding to incident light, said method comprising the steps of:preparing an alkali metal generating device according to claim 6, as asource of the alkali metal; heating the alkali metal generating agenthoused in the case of the alkali metal generating device; and guidingthe alkali metal generated by the heating of the alkali metal generatingagent, to an area for formation of the photo-cathode.
 27. A method ofproduction of a secondary-electron emitting surface for emittingsecondary electrons corresponding to an incident electron, said methodcomprising the steps of: preparing an alkali metal generating agentaccording to claim 1, as a source of the alkali metal; heating thealkali metal generating agent; and guiding the alkali metal generated bythe heating of the alkali metal generating agent, to an area forformation of the secondary-electron emitting surface.
 28. A method ofproduction of a secondary-electron emitting surface for emittingsecondary electrons corresponding to an incident electron, said methodcomprising the steps of: preparing an alkali metal generating deviceaccording to claim 6, as a source of the alkali metal; heating thealkali metal generating agent housed in the case of the alkali metalgenerating device; and guiding the alkali metal generated by the heatingof the alkali metal generating agent, to an area for formation of thesecondary-electron emitting surface.
 29. A method of production of anelectron tube comprising at least a photo-cathode comprising an alkalimetal for emitting a photoelectron corresponding to incident light, saidmethod comprising the steps of: preparing an alkali metal generatingagent according to claim 1, as a source of the alkali metal; heating thealkali metal generating agent; and guiding the alkali metal generated bythe heating of the alkali metal generating agent, to an area forformation of the photo-cathode.
 30. A method of production of anelectron tube comprising at least a photo-cathode comprising an alkalimetal for emitting a photoelectron corresponding to incident light, saidmethod comprising the steps of: preparing an alkali metal generatingdevice according to claim 6, as a source of the alkali metal; heatingthe alkali metal generating agent housed in the case of the alkali metalgenerating device; and guiding the alkali metal generated by the heatingof the alkali metal generating agent, to an area for formation of thephoto-cathode.
 31. A method of production of an electron tube accordingto claim 29, wherein said electron tube comprises one selected from aphotomultiplier tube, a photo-tube, an image tube, and a streak tube.32. A method of production of an electron tube comprising an electronmultiplying part comprised of one or more dynodes each having asecondary-electron emitting surface for emitting secondary electronscorresponding to an incident electron, said method comprising the stepsof: preparing an alkali metal generating agent according to claim 1, asa source of the alkali metal; heating the alkali metal generating agent;and guiding the alkali metal generated by the heating of the alkalimetal generating agent, to an area for formation of thesecondary-electron emitting surface.
 33. A method of production of anelectron tube comprising an electron multiplying part comprised of oneor more dynodes each having a secondary-electron emitting surface foremitting secondary electrons corresponding to an incident electron, saidmethod comprising the steps of: preparing an alkali metal generatingdevice according to claim 6, as a source of the alkali metal; heatingthe alkali metal generating agent housed in the case of the alkali metalgenerating device; and guiding the alkali metal generated by the heatingof the alkali metal generating agent, to an area for formation of thesecondary-electron emitting surface.
 34. A method of production of anelectron tube according to claim 32, wherein said electron tubecomprises one selected from a photomultiplier tube, an image tube, and astreak tube.